The present invention relates to improvements in wicks for wet bulb boxes of humidity sensing devices such as hygrometers, RTD sensors and vapor tension sensors, particularly for use in corrosive environments.
In the art of drying lumber, the lumber is dried in a kiln to remove water vapor. The process is monitored using a wet bulb and dry bulb temperature sensors to indicate the conditions of the atmosphere in the kiln. Typically, the humidity and temperature are carefully monitored and controlled to dry the lumber at a selected rate to avoid damaging the lumber. For example, too-rapid drying can cause the lumber to crack or split, but drying too slow is, of course, inefficient.
Wet bulb sensors are well-known. One type is a hygrometer consisting of two temperature sensors for measuring the temperature of an ambient atmosphere. One directly measures the temperature, while the other is kept wet by a wick, so that the cooling effects of evaporating water will cause a temperature difference between the wet and dry sensors. The lower the humidity of the atmosphere, the greater the drying rate and temperature difference. A particularly common device for controlling the rate of lumber drying is a vapor tension controller. In such a controller the wet bulb is provided in the form of a chamber filled with a preselected, pure gas. The chamber is connected via a capillary to controller, so that as the gas in the chamber contrasts or expands with temperature fluctuations, this is communicated to the controller through the capillary. One known controller of this type is made by The Coe Manufacturing Company, Paynesville, Ohio.
However, in drying lumber, the vapors emanating from the lumber include not only water vapor, but also acids. The acids dissolve in the water of the wick of the wet bulb sensor. This problem has been known a long time. Attempts to solve it have taken the form of providing protective coatings on the temperature sensors, but these have not proven to provide sufficiently long-lasting protection. When the corrosion eats through the wet bulb, changes in the gas pressure are no longer accurately communicated to the controller. Repair requires not only replacing the bulb, but also fully evacuating the chamber and some controller components and replacing the pure gas, all of which is quite expensive.
The wicks which are placed on the wet bulb sensors are replaced rather frequently so they can adequately maintain the wet bulb condition. If the wicks become crusty, slimy, torn or otherwise perforated, they lose their effectiveness and transmit a false signal to the wet bulb. The rate of replacement of wicks varies, depending on a number of conditions. For example, in a setting where the water supply is hard, mineral deposits build up quickly on the wick, requiring rapid replacement. Similarly, it is believed that the acids in the atmosphere attack the wick itself, causing its early failure. Because of this and the fact that wicks are inexpensive, they are considered to be short-term, disposable commodities.
Replacement of the worn-out wick in a hot kiln is not a pleasant job. This is particularly so if the temperature sensor has a bit of age, so that corrosion pits and roughens its surface, snagging the wicks.
As a result, those of ordinary skill in the art have a need for an improved wet bulb wick to reduce corrosion of the wet bulb sensor, permit easy installation of a new wick and lengthen the lifetime of wicks.